This invention relates to a speech control circuit and, in particular, relates to such a circuit which is utilized in a handsfree telephone or a loudspeaker telephone and which provides the correct switching between a transmission mode and a reception mode of the telephone in spite of the presence of an acoustic coupling or a leakage coupling between a transmission side and a reception side.
A handsfree telephone or a loudspeaker telephone has a transmission channel which transmits a transmission speech signal from a microphone to a line such as a telephone line, a reception channel which receives a reception speech signal from the line and energizes a speaker, and a hybrid circuit which couples the transmission channel and the reception channel to the line. As is well known, the handsfree telephone needs to operate in either one of a transmission mode or a reception mode.
A prior speech control circuit for controlling switching between the two modes is explained below with reference to FIG. 1.
In this figure, the symbol M is a microphone, SC.sub.T is a transmission switching circuit having a switching control input which is inserted into a transmission channel which is defined as a line between (a) and (b), SP is a speaker, SC.sub.R is a reception switching circuit having a switching control input which is inserted into a reception channel which is defined as a line between (c) and (d), and HYB is a hybrid circuit which couples the transmission channel and the reception channel to a line such as a telephone line. The block surrounded by the dotted line is a speech control circuit for controlling switching actions of the transmission switching circuit SC.sub.T and reception switching circuit SC.sub.R in accordance with the speech level in each channel.
The speech control circuit is composed of a transmission amplifier A.sub.T, a reception amplifier A.sub.R, a signal detector SD, a time constant circuit TC, and a driver circuit DC.
The output of microphone M is connected to the input of the transmission amplifier A.sub.T for amplifying the transmission speech signal from the microphone M, the output of which is connected to one input of the signal detector SD. The output (c) of the hybrid circuit HYB is connected to the input of the reception amplifier A.sub.R from the line through the hybrid circuit HYB, the output of which is connected to the other input of the signal detector SD. The voltage V.sub.t of the transmission amplifier A.sub.T and the voltage V.sub.r of the reception amplifier A.sub.B are compared with each other by the signal detector SD in order to determine the operational mode of the handsfree telephone.
In the comparison operation, if the voltage V.sub.t is higher than the voltage V.sub.r, an output pulse in the form of square-wave is produced, thus, the operational mode is determined to be the transmission mode. On the other hand, when the voltage V.sub.r is higher than the voltage V.sub.t, no output pulse is produced, and then the operational mode is determined to be the reception mode. When V.sub.r =V.sub.t =0 is satisfied, the operational mode is the reception mode.
The output of the signal detector SD is connected to the input of the time constant circuit TC for elongating a pulse-width of an output pulse from the signal detector SD by a predetermined time constant in order to prevent the frequent switching between the transmission mode and the reception mode. The output of the time constant circuit TC is connected to the driver circuit DC having two outputs, one of which is connected to the switching control input of the transmission switching circuit SC.sub.T, the other of which is connected through a inverter INV to the switching control input of the reception switching circuit SC.sub.R.
The driver circuit DC controls the transmission switching circuit SC.sub.T and the reception switching circuit SC.sub.R in accordance with the output signal of the time constant circuit TC so that either one of two switching circuits is ON, and the other is OFF. On the one hand, when an elongated signal output pulse from the time constant circuit TC is supplied to the input of the driver circuit DC, this pulse is supplied to the switching control input of the transmission switching circuit SC.sub.T, thereby causing it to be ON. Therefore, a transmission speech signal from the microphone M is transmitted to the line through the hybrid circuit HYB. Simultaneously, that pulse is inverted by the inverter INV and supplied to the switching control input of the reception switching circuit SC.sub.R, thereby causing it to be OFF. Therefore, a reception speech signal from the line is not applied to the speaker SP. On the other hand, when there exists no output pulse from the time constant circuit TC at the input of the driver circuit DC, it will be apparent from the above description that the transmission switching circuit SC.sub.T is OFF and the reception switching circuit SC.sub.R is ON.
Accordingly, switching between two modes is provided by the speech control circuit.
However, the prior circuit as mentioned above has the following disadvantages because of the leakage coupling in the hybrid circuit HYB and/or the acoustic coupling between the microphone M and the speaker SP.
Assuming that the hybrid circuit HYB is an ideal one, in the transmission mode, the transmission signal from the microphone M would be perfectly transmitted to the line through the circuit HYB. However, in an actual hybrid circuit, there exists a leakage coupling (as shown by the symbol A in FIG. 1) between the transmission channel and the reception channel resulting from the impedance unbalance of this circuit. Accordingly, some of the transmission signal leaks to the reception channel through the hybrid circuit HYB with some delay time. This leakage signal appearing in the reception channel is called "a false speech signal". Such a false speech signal is then amplified by the reception amplifier A.sub.R. At this time, suppose that the true speech transmission signal is amplified by the transmission amplifier A.sub.T and then supplied to the signal detector SD. In this case, if the level of the false speech signal in the reception channel exceeds that of the true speech transmission signal, since the voltage V.sub.r resulting from the false speech signal is larger than the voltage V.sub.t, the operational mode would change from the transmission mode to the reception mode, although the correct operational mode is the transmission mode. In particular, the incorrect switching resulting from a false speech signal which appears in the reception channel with some delay time after disappearance of a transmission speech signal in the transmission channel can not be avoided at all in this configuration. These incorrect switching actions are often observed in practical use.
On the other hand, in the reception mode, a reception signal from the line through the hybrid circuit HYB is converted to an acoustic signal by the speaker SP. The acoustic signal is then applied with some delay time to the microphone M by an acoustic coupling (as shown by the symbol B in FIG. 1), and is converted to the electric speech signal by the microphone M, which is also called the false speech signal. This false speech signal is amplified by the transmission amplifier A.sub.T and supplied to the signal detector SD. At this time, suppose that the reception signal is amplified by the reception amplifier A.sub.R. Under this condition, if the voltage V.sub.t resulting from the false speech signal is higher than the voltage V.sub.r, the operational mode would change from the reception mode to the transmission mode, although the correct operational mode is the reception mode. In particular, the incorrect operation resulting from a false speech signal which appears with some delay time in the transmission channel after disappearance of a reception speech signal in the reception channel can not be avoided at all in this configuration. These incorrect operations are very often observed, in particular, in a handsfree telephone in noisy places such as a car, since a sound level of an acoustic signal from the speaker must be made very high.
Therefore, under these conditions, it is impossible for the circuit of FIG. 1 to provide the correct switching between two modes in accordance with the result of the comparison of the level of the transmission signal with that of the reception signal.